Garnet

Garnet
GarnetCrystalUSGOV.jpg
General
Category nesosilicates
Chemical formula The general formula X3Y2(SiO4)3
Identification
Color virtually all colors
Crystal habit rhombic dodecahedra or cubic
Crystal system Cubic
Cleavage None
Fracture conchoidal to uneven
Mohs Scale hardness 6.0 - 7.5
Luster vitreous to resinous
Polish luster vitreous to subadamantine [1]
Refractive index 1.72 - 1.94
Optical Properties Single refractive, often anomalous double refractive [1]
Birefringence None
Pleochroism only in certain rare varieties
Streak White
Specific gravity 3.1 - 4.3
Major varieties
Pyrope Mg3Al2Si3O12
Almandine Fe3Al2Si3O12
Spessartite Mn3Al2Si3O12
Andradite Ca3Fe2Si3O12
Grossular Ca3Al2Si3O12
Uvarovite Ca3Cr2Si3O12

The garnet group includes a group of minerals that have been used since the Bronze Age as gemstones and abrasives. The name "garnet" comes from the Latin granatus ("grain"), possibly a reference to the Punica granatum ("pomegranate"), a plant with red seeds similar in shape, size, and color to some garnet crystals.

Six common species of garnet are recognized based on their chemical composition. They are pyrope, almandine, spessartite, grossular (varieties of which are hessonite or cinnamon-stone and tsavorite), uvarovite and andradite. The garnets make up two solid solution series: 1. pyrope-almandine-spessarite and 2. uvarovite-grossular-andradite.

Contents

Physical properties

Properties

Garnets species are found in many colors including red, orange, yellow, green, blue, purple, brown, black, pink and colorless. The rarest of these is the blue garnet, discovered in the late 1990s in Bekily, Madagascar. It is also found in parts of the United States, Russia and Turkey. It changes color from blue-green in the daylight to purple in incandescent light, as a result of the relatively high amounts of vanadium (about 1 wt.% V2O3). Other varieties of color-changing garnets exist. In daylight, their color ranges from shades of green, beige, brown, gray, and blue, but in incandescent light, they appear a reddish or purplish/pink color. Because of their color changing quality, this kind of garnet is often mistaken for Alexandrite.

Garnet species’s light transmission properties can range from the gemstone-quality transparent specimens to the opaque varieties used for industrial purposes as abrasives. The mineral’s luster is categorized as vitreous (glass-like) or resinous (amber-like).

Crystal structure

Molecular model of garnet.

Garnets are nesosilicates having the general formula X3Y2(SiO4)3. The X site is usually occupied by divalent cations (Ca2+, Mg2+, Fe2+) and the Y site by trivalent cations (Al3+, Fe3+, Cr3+) in an octahedral/tetrahedral framework with [SiO4]4− occupying the tetrahedra.[2] Garnets are most often found in the dodecahedral crystal habit, but are also commonly found in the trapezohedron habit. (Note: the word "trapezohedron" as used here and in most mineral texts refers to the shape called a Deltoidal icositetrahedron in solid geometry.) They crystallize in the cubic system, having three axes that are all of equal length and perpendicular to each other. Garnets do not show cleavage, so when they fracture under stress, sharp irregular pieces are formed.

Hardness

Because the chemical composition of garnet varies, the atomic bonds in some species are stronger than in others. As a result, this mineral group shows a range of hardness on the Mohs Scale of about 6.5 to 7.5. The harder species, like almandine, are often used for abrasive purposes.

Garnet group endmember species

Pyralspite garnets - Aluminium in Y site

Almandine

Almandine in metamorphic rock

Almandine, sometimes incorrectly called almandite, is the modern gem known as carbuncle (though originally almost any red gemstone was known by this name). The term "carbuncle" is derived from the Latin meaning "live coal" or burning charcoal. The name Almandine is a corruption of Alabanda, a region in Asia Minor where these stones were cut in ancient times. Chemically, almandine is an iron-aluminium garnet with the formula Fe3Al2(SiO4)3; the deep red transparent stones are often called precious garnet and are used as gemstones (being the most common of the gem garnets). Almandine occurs in metamorphic rocks like mica schists, associated with minerals such as staurolite, kyanite, andalusite, and others. Almandine has nicknames of Oriental garnet, almandine ruby, and carbuncle.

Pyrope

Pyrope (from the Greek pyrōpós meaning "fire-eyed") is red in color and chemically a magnesium aluminium silicate with the formula Mg3Al2(SiO4)3, though the magnesium can be replaced in part by calcium and ferrous iron. The color of pyrope varies from deep red to almost black. Transparent pyropes are used as gemstones.

A variety of pyrope from Macon County, North Carolina is a violet-red shade and has been called rhodolite, from the Greek meaning "a rose." In chemical composition it may be considered as essentially an isomorphous mixture of pyrope and almandite, in the proportion of two parts pyrope to one part almandite. Pyrope has tradenames some of which are misnomers; Cape ruby, Arizona ruby, California ruby, Rocky Mountain ruby, and Bohemian garnet from the Czech Republic. Another intriguing find is the blue color-changing garnets from Madagascar, a pyrope spessartine mix. The color of these blue garnets is not like sapphire blue in subdued daylight but more reminiscent of the grayish blues and greenish blues sometimes seen in spinel. However, in white LED light the color is equal to the best cornflower blue sapphire, or D block tanzanite; this is due to the blue garnet's ability to absorb the yellow component of the emitted light.

Pyrope is an indicator mineral for high-pressure rocks. The garnets from mantle derived rocks, peridotites and eclogites, commonly contain a pyrope variety.

Spessartine

Spessartine (the reddish mineral)

Spessartine or incorrectly spessartite is manganese aluminium garnet, Mn3Al2(SiO4)3. Its name is derived from Spessart in Bavaria. It occurs most often in granite pegmatite and allied rock types and in certain low grade metamorphic phyllites. Spessartine of a beautiful orange-yellow is found in Madagascar (see Mandarin garnet). Violet-red spessartines are found in rhyolites in Colorado and Maine.

Ugrandite group - calcium in X site

Andradite

Andradite (the black mineral)

Andradite is a calcium-iron garnet, Ca3Fe2(SiO4)3, is of variable composition and may be red, yellow, brown, green or black. The recognized varieties are topazolite (yellow or green), demantoid (green) and melantite (black). Andradite is found both in deep-seated igneous rocks like syenite as well as serpentines, schists, and crystalline limestone. Demantoid has been called the "emerald of the Urals" from its occurrence there, and is one of the most prized of garnet varieties. Topazolite is a golden yellow variety and melanite is a black variety.

Grossular

Grossular on display at the US National Museum of Natural History.

Grossular is a calcium-aluminium garnet with the formula Ca3Al2(SiO4)3, though the calcium may in part be replaced by ferrous iron and the aluminium by ferric iron. The name grossular is derived from the botanical name for the gooseberry, grossularia, in reference to the green garnet of this composition that is found in Siberia. Other shades include cinnamon brown (cinnamon stone variety), red, and yellow. Because of its inferior hardness to zircon, which the yellow crystals resemble, they have also been called hessonite from the Greek meaning inferior. Grossular is found in contact metamorphosed limestones with vesuvianite, diopside, wollastonite and wernerite.

One of the most sought after varieties of gem garnet is the fine green grossular garnet from Kenya and Tanzania called tsavorite. This garnet was discovered in the 1960s in the Tsavo area of Kenya, from which the gem takes its name.

Uvarovite

Uvarovite

Uvarovite is a calcium chromium garnet with the formula Ca3Cr2(SiO4)3. This is a rather rare garnet, bright green in color, usually found as small crystals associated with chromite in peridotite, serpentinite, and kimberlites. It is found in crystalline marbles and schists in the Ural mountains of Russia and Outokumpu, Finland.

Less common species

Knorringite

Knorringite is a magnesium chromium garnet species with the formula Mg3Cr2(SiO4)3. Pure endmember knorringite never occurs in nature. Knorringite is only formed under high pressure and is often found in kimberlites. It is used as an indicator mineral in the search for diamonds.

Synthetic garnets

In yttrium iron garnet (YIG), Y3Fe2(FeO4)3, the five iron(III) ions occupy two octahedral and three tetrahedral sites, with the yttrium(III) ions coordinated by eight oxygen ions in an irregular cube. The iron ions in the two coordination sites exhibit different spins, resulting in magnetic behaviour. YIG is a ferrimagnetic material having a Curie temperature of 550 K. By substituting specific sites with rare earth elements, for example, interesting magnetic properties can be obtained.

One example for this is gadolinium gallium garnet, Gd3Ga2(GaO4)3, which is synthesized for use in magnetic bubble memory.

Yttrium aluminium garnet (YAG), Y3Al2(AlO4)3, is used for synthetic gemstone. When doped with neodymium (Nd3+), these YAl-garnets are useful as the lasing medium in lasers.

Geological importance of garnet

The Garnet group is a key mineral in interpreting the genesis of many igneous and metamorphic rocks via geothermobarometry. Diffusion of elements is relatively slow in garnet compared to rates in many other minerals, and garnets are also relatively resistant to alteration. Hence, individual garnets commonly preserve compositional zonations that are used to interpret the temperature-time histories of the rocks in which they grew. Garnet grains that lack compositional zonation commonly are interpreted as having been homogenized by diffusion, and the inferred homogenization also has implications for the temperature-time history of the host rock.

Garnets are also useful in defining metamorphic facies of rocks. For instance, eclogite can be defined as a rock of basalt composition, but mainly consisting of garnet and omphacite. Pyrope-rich garnet is restricted to relatively high-pressure metamorphic rocks, such as those in the lower crust and in the Earth's mantle. Peridotite may contain plagioclase, or aluminium-rich spinel, or pyrope-rich garnet, and the presence of each of the three minerals defines a pressure-temperature range in which the mineral could equilibrate with olivine plus pyroxene: the three are listed in order of increasing pressure for stability of the peridotite mineral assemblage. Hence, garnet peridotite must have been formed at great depth in the earth. Xenoliths of garnet peridotite have been carried up from depths of 100 km and greater by kimberlite, and garnets from such disaggegated xenoliths are used as a kimberlite indicator minerals in diamond prospecting. At depths of about 300 to 400 km and greater, a pyroxene component is dissolved in garnet, by the substitution of (Mg,Fe) plus Si for 2Al in the octahedral (Y) site in the garnet structure, creating unusually silica-rich garnets that have solid solution towards majorite. Such silica-rich garnets have been identified as inclusions within diamonds.

Uses of garnets

Pendant in uvarovite, a rare bright-green garnet.

Pure crystals of garnet are used as gemstones. Garnet sand is a good abrasive, and a common replacement for silica sand in sand blasting. Mixed with very high pressure water, garnet is used to cut steel and other materials in water jets. Garnet sand is also used for water filtration media.

Abrasive garnet

As an abrasive garnet can be broadly divided in two categories, blasting grade and water jet grade. The garnet, as it is mined and collected, is crushed to finer grains; all pieces which are larger than 60 mesh (250 microns) are normally used for sand blasting. The pieces between 60 mesh (250 microns) and 200 mesh (74 microns) are normally used for water jet cutting. The remaining garnet pieces that are finer than 200 mesh (74 microns) are used for glass polishing and lapping. Regardless of the application, the larger grain sizes are used for faster work and the smaller ones are used for finer finishes. There are different kinds of abrasive garnets which can be divided based on their origin.

Beach garnet is beach sand which is quite abundant on Indian and Australian coasts. This is the largest source of abrasive garnet today. This material is particularly popular due to its consistent supplies, huge quantities and clean material. The common problems with this material are the presence of ilmenite and chloride compounds. Producers from India as well as Australia treat the garnet to remove the impurities from the material. Another limitation of the garnet from these sources is the inability to obtain coarser grains. Since the material is being naturally crushed and ground on the beaches for past centuries, the material is normally available in fine sizes only. Most of the garnet at the Tuticorin beach is 80 mesh, and ranges from 56 mesh to 100 mesh size.

River garnet is particularly abundant in Australia. Over the years the company has earned a good reputation for producing high quality garnet in fine sizes. The river sand garnet is a placer deposit; it is garnet eroded from garnet bearing rocks, crushed and powdered by natural processes and later blown away by wind. There are few known deposits of this type. GMA is the one of the world's largest companies producing and distributing garnet, which owns many of the river garnet mines in Australia. Besides GMA's mines there are a few other such deposits throughout the world, but none match the size of GMA's.

Rock garnet is perhaps the garnet type used for the longest period of time. This type of garnet is produced in America, in China and in western India. Barton Mines in the United States consists of rock garnet mines and is the oldest known garnet mine in the world. This type of garnet is the least abundant of the various sources, but is the preferred source of garnet. Hexagonal crystals of garnet the size of basketballs can be found in certain parts of western India. These crystals are crushed in mills and then purified by wind blowing, magnetic separation, sieving and if required washing. Being freshly crushed this garnet has the sharpest edges and therefore performs far better than other kinds of garnet. Both the river and the beach garnet suffer from the tumbling effect of hundred of thousands of years which rounds off the edges. In contrast, rock garnet is quite sharp and more efficient since it is freshly ground.

Garnet has been mined in western Rajasthan for the past 200 years, but mainly for the gemstone grade stones. Mines in Rajmahal are famous for producing some of the finest quality garnets. The history of abrasive garnet from this area began about 50 years ago. It was mainly mined as a secondary product while mining for gem garnets and was used as lapping and polishing media for the glass and bottle industries. The potential was realized by Dr. Gagan Goyal, who theorized that since it is rock garnet, it would perform much better than beach sand and river garnet. Today there is a tremendous demand for Rajasthan garnet in the sandblasting as well as the water jet cutting industry, but the total production is still low. The host rock of the garnet here is mica schist and quartz and the total percentage of garnet is not more than 7% to 10%, which makes the material extremely costly and non economical to extract for non-gemstone applications. Another problem with garnet from this area is the dust. No amount of cleaning or washing can make the material completely free of dust.

Gemstone

The garnet gemstone varieties occur in shades of green, red, yellow and orange. The same range of variation can be seen in its prices as well. [5]

References

  1. ↑ 1.0 1.1 1.2 Gemological Institute of America, GIA Gem Reference Guide 1995, ISBN 0-87311-019-6
  2. ↑ Smyth, Joe. "Mineral Structure Data". Garnet. University of Colorado. Retrieved on 2007-01-12.
  3. ↑ State of Connecticut, Sites º Seals º Symbols; Connecticut State Register & Manual; retrieved on January 4, 2007
  4. ↑ New York State Gem; State Symbols USA; retrieved on October 12, 2007
  5. ↑ Geological Sciences at University of Texas, Austin
  • Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0-471-80580-7
  • Color Encyclopedia of Gemstones ISBN 0-442-20333-0

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